Commit message: ax25: fix UAF bug in ax25_send_control()

There are UAF bugs in ax25_send_control(), when we call ax25_release()
to deallocate ax25_dev. The possible race condition is shown below:

      (Thread 1)              |     (Thread 2)
ax25_dev_device_up() //(1)    |
                              | ax25_kill_by_device()
ax25_bind()          //(2)    |
ax25_connect()                | ...
 ax25->state = AX25_STATE_1   |
 ...                          | ax25_dev_device_down() //(3)

      (Thread 3)
ax25_release()                |
 ax25_dev_put()  //(4) FREE   |
 case AX25_STATE_1:           |
  ax25_send_control()         |
   alloc_skb()       //USE    |

The refcount of ax25_dev increases in position (1) and (2), and
decreases in position (3) and (4). The ax25_dev will be freed
before dereference sites in ax25_send_control().

The following is part of the report:

[  102.297448] BUG: KASAN: use-after-free in ax25_send_control+0x33/0x210
[  102.297448] Read of size 8 at addr ffff888009e6e408 by task ax25_close/602
[  102.297448] Call Trace:
[  102.303751]  ax25_send_control+0x33/0x210
[  102.303751]  ax25_release+0x356/0x450
[  102.305431]  __sock_release+0x6d/0x120
[  102.305431]  sock_close+0xf/0x20
[  102.305431]  __fput+0x11f/0x420
[  102.305431]  task_work_run+0x86/0xd0
[  102.307130]  get_signal+0x1075/0x1220
[  102.308253]  arch_do_signal_or_restart+0x1df/0xc00
[  102.308253]  exit_to_user_mode_prepare+0x150/0x1e0
[  102.308253]  syscall_exit_to_user_mode+0x19/0x50
[  102.308253]  do_syscall_64+0x48/0x90
[  102.308253]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  102.308253] RIP: 0033:0x405ae7

This patch defers the free operation of ax25_dev and net_device after
all corresponding dereference sites in ax25_release() to avoid UAF.

Fixes: 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
CVE: CVE-2022-1204
CWE: CWE-416
Vulnerability description: A use-after-free flaw was found in the Linux kernel’s Amateur Radio AX.25 protocol functionality in the way a user connects with the protocol. This flaw allows a local user to crash the system.

Project: linux
File: af_ax25.c

static int ax25_release(struct socket *sock)
{
	struct sock *sk = sock->sk;
	ax25_cb *ax25;
	ax25_dev *ax25_dev;

	if (sk == NULL)
		return 0;

	sock_hold(sk);
	lock_sock(sk);
	sock_orphan(sk);
	ax25 = sk_to_ax25(sk);
	ax25_dev = ax25->ax25_dev;
	if (ax25_dev) {
		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
		ax25_dev_put(ax25_dev);
	}

	if (sk->sk_type == SOCK_SEQPACKET) {
		switch (ax25->state) {
		case AX25_STATE_0:
			release_sock(sk);
			ax25_disconnect(ax25, 0);
			lock_sock(sk);
			ax25_destroy_socket(ax25);
			break;

		case AX25_STATE_1:
		case AX25_STATE_2:
			ax25_send_control(ax25, AX25_DISC, AX25_POLLON, AX25_COMMAND);
			release_sock(sk);
			ax25_disconnect(ax25, 0);
			lock_sock(sk);
			if (!sock_flag(ax25->sk, SOCK_DESTROY))
				ax25_destroy_socket(ax25);
			break;

		case AX25_STATE_3:
		case AX25_STATE_4:
			ax25_clear_queues(ax25);
			ax25->n2count = 0;

			switch (ax25->ax25_dev->values[AX25_VALUES_PROTOCOL]) {
			case AX25_PROTO_STD_SIMPLEX:
			case AX25_PROTO_STD_DUPLEX:
				ax25_send_control(ax25,
						  AX25_DISC,
						  AX25_POLLON,
						  AX25_COMMAND);
				ax25_stop_t2timer(ax25);
				ax25_stop_t3timer(ax25);
				ax25_stop_idletimer(ax25);
				break;
#ifdef CONFIG_AX25_DAMA_SLAVE
			case AX25_PROTO_DAMA_SLAVE:
				ax25_stop_t3timer(ax25);
				ax25_stop_idletimer(ax25);
				break;
#endif
			}
			ax25_calculate_t1(ax25);
			ax25_start_t1timer(ax25);
			ax25->state = AX25_STATE_2;
			sk->sk_state                = TCP_CLOSE;
			sk->sk_shutdown            |= SEND_SHUTDOWN;
			sk->sk_state_change(sk);
			sock_set_flag(sk, SOCK_DESTROY);
			break;

		default:
			break;
		}
	} else {
		sk->sk_state     = TCP_CLOSE;
		sk->sk_shutdown |= SEND_SHUTDOWN;
		sk->sk_state_change(sk);
		ax25_destroy_socket(ax25);
	}

	sock->sk   = NULL;
	release_sock(sk);
	sock_put(sk);

	return 0;
}